(1) Field of the Invention
The present invention relates to a polymer electrolyte battery and more specifically to a polymer electrolyte battery containing a positive electrode and a negative electrode that sandwich a polyolefine porous membrane containing a gel polymer electrolyte.
(2) Description of the Related Art
Portable devices, such as a portable phone, an audio-video player, a digital camera, and a personal digital assistant, are now in increasing demand. In response to this demand, the need for a thin, light-weight battery with a high capacity rapidly increases. With its ultra thinness and light weight, a polymer electrolyte battery is suitable to be loaded in a portable device and expected to respond to the increasing need.
A polymer electrolyte battery contains a membrane made of a polymer electrolyte between a positive electrode plate and a negative electrode plate, and has an advantage of not causing a liquid electrolyte leak unlike other batteries using a liquid electrolyte.
A polymer electrolyte battery often uses a lithium complex oxide as a positive electrode active material. For a negative electrode material, lithium metal and an aluminum-lithium alloy have been conventionally used. With a battery using such negative electrode material, however, a dendrite is likely to grow as a result of the battery being repeatedly charged and discharged. Accordingly, a carbon material capable of lithium ion occlusion and release is now often used as a negative electrode material.
As a polymer electrolyte, a solid electrolyte containing a polyalkylene oxide in which a solute is dissolved is conventionally known. A containing alkylene oxide polymer, for instance, but has low ionic conductivity. This conventional solid electrolyte therefore has a drawback in that it has a small high rate discharge capacity.
To overcome this disadvantage, a polymer electrolyte battery containing a gel polymer electrolyte is developed. This gel polymer electrolyte contains a polymer that is produced by curing a polymer precursor such as polyalkylene glycol diacrylate. As the gel polymer electrolyte has higher ion conductivity than a solid electrolyte, a polymer electrolyte battery containing such gel polymer electrolyte can achieve a relatively high high-rate discharge capacity.
When this gel polymer electrolyte is included in a polyolefine porous membrane to be inserted between the positive electrode and the negative electrode, a polymer electrolyte battery having an improved mechanical strength can be developed.
For a thin-type polymer electrolyte battery, its external casing is usually formed by combining soft sheet members, such as laminated aluminum, together. As such external casing can be easily deformed (dent or bent) due to an external force, a thin-type polymer electrolyte battery is required to have a mechanical strength to maintain good battery performance even when a minor deformation occurs to the battery.
At the same time, as batteries are now used in a variety of types of apparatuses, they may be used at a very high temperature. Accordingly, a battery is also required to have a high heat resistance to prevent an internal short circuit from occurring at a high temperature.
For the above conventional polymer electrolyte battery which has a polyolefine porous membrane containing a gel polymer electrolyte, however, such internal short circuit is likely to occur at around 150xc2x0 C. although the polymer electrolyte battery has a relatively high heat resistance. The above polyolefine porous membrane, which is usually used as a porous membrane, shrinks at a high temperature so that the positive electrode and the negative electrode become likely to be in contact with each other. This is considered to be the cause of an internal short circuit.
The present invention aims to provide a polymer electrolyte battery having a high discharge capacity, a high mechanical strength, and a high heat resistance.
To achieve this object, the present invention is applied to a polymer electrolyte battery including an electrode unit that contains a positive electrode plate and a negative electrode plate, into which a porous membrane containing a polymer electrolyte is inserted. Around the circumference of the electrode unit of the present battery, a polymer electrolyte material covers an edge part of the positive electrode plate and an edge part of the porous membrane together, and the edge part of the porous membrane and an edge part of the negative electrode plate together.
The porous membrane inserted between the positive electrode plate and the negative electrode plate allows the above polymer electrolyte battery to have a high mechanical strength.
The construction of the above polymer electrolyte battery also prevents a short circuit from occurring inside the polymer electrolyte battery due to the following reason.
At a high temperature around 150xc2x0 C., an internal short circuit is likely to occur to a conventional polymer electrolyte battery that includes a polyolefine porous membrane containing a gel polymer electrolyte. This is because the polyolefine porous membrane, which is usually used as a porous membrane, shrinks and deforms due to a high temperature, so that the positive electrode plate and the negative electrode plate become likely to be in contact with one another.
For the polymer electrolyte battery of the present invention, however, a polymer electrolyte material connects an edge part of the porous membrane with an edge part of the positive electrode plate and with an edge part of the negative electrode plate. This is to say, the polymer electrolyte material holds edge parts of: the positive electrode plate; the negative electrode plate; and the porous membrane. This allows the porous membrane to keep its original shape against the contraction force, thereby preventing a short circuit from occurring inside the polymer electrolyte battery.
Here, it is preferable to use a polymer electrolyte of the same type for the above electrolyte material and for the polymer electrolyte that is contained in the porous membrane. This facilitates production process of the polymer electrolyte battery, and does not have any negative effect on battery performance.
For a polymer electrolyte battery that uses an external casing produced by combining sheet members together such as a laminated aluminum film, the external casing is likely to deform due to an external force, or swell due to an increase in an internal pressure when an internal short circuit occurs. Accordingly, providing a high mechanical strength and a high heat resistance to this type of polymer electrolyte battery has great practical importance.
The above polymer electrolyte battery can be easily produced through the following processes. The electrode unit is produced by inserting a porous membrane between a positive electrode plate and a negative electrode plate. The produced electrode unit is impregnated with a pregel solution composed of a liquid electrolyte and a polymer precursor. The pregel solution that is added into and affixed around the electrode unit is then cured.